1. Field of the Invention
The present invention relates to multi-chip modules and, more specifically, to multi-chip modules including semiconductor devices that are mounted to one another in a back-to-back relationship. The present invention also relates to chip-on-board assemblies. Particularly, the present invention relates to bare and minimally packaged semiconductor devices which are mountable substantially perpendicularly to a carrier substrate such as a printed circuit board. All of the bond pads of the semiconductor devices are disposed proximate a single edge or side thereof. The present invention also relates to methods and devices for securing back-to-back mounted semiconductor devices perpendicularly relative to a carrier substrate and for establishing electrical connections between the semiconductor devices and the carrier substrate to fabricate a multi-chip module.
2. Background of Related Art
The direct attachment of an unpackaged semiconductor device to a circuit board is known in the art as chip-on-board technology. Semiconductor devices that are directly (i.e., without packaging) mountable to a circuit board typically include peripherally disposed bond pads that are adjacent more than one edge thereof or in an area array over the active surface of the semiconductor device. Methods for attaching unpackaged or minimally packaged (such as the so-called “chip scale package”) semiconductor devices directly to a circuit board include wire bonding, flip-chip technology and tape automated bonding. Typically, when such techniques are employed, a semiconductor device (typically a singulated semiconductor die) which includes bond pads on an active surface thereof is oriented over the circuit board, either active surface up or active surface down, and substantially parallel thereto, in order to establish an electrical connection between the semiconductor device and the circuit board by one of the aforementioned techniques. After electrically connecting such a semiconductor device to a circuit board, a protective coating may be applied over the semiconductor device.
However, the active surface-down placement of a semiconductor device directly against a circuit board is somewhat undesirable in that, due to the substantially parallel orientation of the semiconductor device relative to the circuit board and the location of an integrated circuit on the active surface of the semiconductor device against the circuit board, heat must pass through the circuit board or through the substrate of the semiconductor device in order to dissipate from the active surface of the semiconductor device. Thus, the transfer of heat away from the semiconductor device is relatively slow and may result in the generation of damaging ambient temperatures at the active surface during prolonged operation of the semiconductor device. A horizontal (parallel) orientation of the semiconductor device relative to the circuit board also causes the semiconductor device to consume a great deal of area or “real estate” on the circuit board. Moreover, conventional chip-on-board attachments as previously referenced are typically permanent, making them somewhat undesirable from the standpoint that they are not readily user-upgradable by substitution of different, higher-performance semiconductor devices.
Various multi-chip module arrangements have been developed to conserve “real estate” on a carrier substrate. Some multi-chip modules include a plurality of mutually parallel bare semiconductor devices that are secured in a stack, one above another. The semiconductor devices of stacked multi-chip modules are typically substantially parallel to the carrier substrate to which they are secured. Multi-chip modules with vertically oriented dice are also known. Exemplary device stack arrangements are described in the following United States patents: U.S. Pat. No. 5,291,061, issued to Ball on Mar. 1, 1994; and U.S. Pat. No. 5,323,060 issued to Fogal et al. on Jun. 21, 1994. Exemplary vertical dice arrangements are described in U.S. Pat. No. 5,362,986, issued to Angiulli et al. on Nov. 8, 1994; and U.S. Pat. No. 5,397,747, issued to Angiulli et al. on Mar. 14, 1995.
While multi-chip modules successfully conserve some of the “real estate” on a carrier substrate, the horizontal (i.e., substantially parallel to the carrier substrate) orientation of some such devices still consumes a significant area thereof. Moreover, the stacking of many such semiconductor devices inhibits the dissipation of heat from the devices of the stack, which may, as noted above, adversely affect the performance of the semiconductor devices, and may even damage them. Finally, many known multi-chip modules are electrically connected to a carrier substrate with solder or other permanent means, such as wire bonds or TAB (tape automated bonding or flex circuit) connections, and are also permanently mechanically attached by solder, epoxy or another bonding agent. Thus, such semiconductor devices are not readily removable from the carrier substrate or readily replaceable thereupon (i.e., they are not user-upgradable).
Similarly, vertical surface mount packages are known in the art. When compared with traditional, horizontally mountable semiconductor packages and chip-on-board devices, many vertical surface mount packages have a superior ability to transfer heat away from the semiconductor device due to exposure of both major surfaces of the package. Vertical surface mount packages also consume less area on a circuit board than a horizontally mounted package of the same size. Thus, many skilled individuals in the semiconductor industry are finding vertical surface mount packages more desirable than their traditional, horizontally mountable counterparts. The following U.S. patents disclose various exemplary vertical surface mount packages: Re. 34,794, issued to Warren M. Farnworth on Nov. 22, 1994; U.S. Pat. No. 5,444,304, issued to Kouija Hara and Jun Tanabe on Aug. 22, 1995; U.S. Pat. No. 5,450,289, issued to Yooung D. Kweon and Min C. An on Sep. 12, 1995; U.S. Pat. No. 5,451,815, issued to Norio Taniguchi et al. on Sep. 19, 1995; U.S. Pat. No. 5,592,019, issued to Tetsuya Ueda et al. on Jan. 7, 1997; and U.S. Pat. No. 5,635,760, issued to Toru Ishikawa on Jun. 3, 1997.
Many vertical surface mount packages are somewhat undesirable in that they include leads which operatively connect a semiconductor device to a circuit board. The leads of such devices tend to increase the impedance and decrease the overall speed with which such devices conduct electrical signals. Moreover, the required packaging of many such devices adds to their undesirability. Typically, packaging requires multiple additional materials and manufacturing steps, which translate into increased production costs, which are further increased due to the potential for package and connection defects and resulting lower final yields. The packaging of many vertical surface mount packages, and thus thickness in excess of that of the packaged die, also tends to consume additional area or “real estate” on the circuit board to which they are attached. Further, the materials of most semiconductor device packages tend to inhibit the transfer of heat from the semiconductor device contained therein. Moreover, many vertical surface mount packages are not readily user-upgradable due to permanent connections to the carrier substrate.
Accordingly, the inventor has recognized a need for a semiconductor device configuration that has low impedance, provides improved heat transfer and conserves “real estate” on a carrier substrate. There is also a need for user-upgradable semiconductor devices, which is addressed by some embodiments of the invention.